The invention relates generally to turbomachines, and more particularly to turbomachines such as steam turbines used in combined cycle power plants having district heating/combined heat-power (DH/CHP) applications.
Combined cycle power plants employ two or more thermodynamic cycles working in tandem to extract useful energy from heat. For example, a gas turbine and a steam turbine may be used in tandem. The gas turbine may burn a fuel, e.g., natural gas or synthesis gas, and exhaust hot gas, which is used to generate steam to power a steam turbine. Similar arrangements may also be used for other applications such as, e.g., marine propulsion and other heat and power applications.
Gas turbines typically operate with a range described herein as the Normal Load Range (“NLR”). The NLR includes the range over which a gas turbine operates during normal operation of a combined cycle power plant. Commonly, NLR is associated with operation in compliance with emissions permitting and grid-code. Certain operations, for example operation associated with plant start-up or shut-down or casualty situations, may be outside the NLR.
Steam turbines typically include sections that operate at predetermined pressure ranges. The steam turbine may include, e.g., a high pressure (HP) section, an intermediate pressure (IP) section, and may further include a low pressure (LP) section. The rotating blades disposed within each of these sections may be mounted on an axial shaft, and collectively make up the rotor. Control valves and intercept valves may control steam flow through the HP and the IP sections, respectively. In an opposed flow steam turbine, the HP section and the IP section may, for example, be disposed such that steam flows in opposite directions as it moves through the stages of each section. Rotor thrust is typically balanced between the opposed sections.
During operation within the NLR of the gas turbine, the combined cycle may export steam to district heating or other heat applications. The NLR may be defined by endpoints of a Maximum Normal Load (“Max-NL”) and a Minimum Normal Load (“Min-NL”). In particular, low pressure steam may be extracted from the steam turbine cycle after it has passed through the IP section, prior to entering the LP section, for use in the particular heat application. In some instances, however, the low pressure steam may be insufficient to meet the heat application's needs. In that instance, intermediate pressure steam may be extracted from the steam turbine prior to its entering the IP section to augment the steam volume provided to the heat application. In opposed flow steam turbines, this extraction of intermediate pressure steam may reduce the thrust in the IP section, and may therefore contribute to a thrust imbalance between the HP and IP sections.